Various optical techniques have been developed to measure a thin film coating on a substrate in the prior art. These optical techniques provide non-invasive means to characterize various types of thin films widely used in industry and academia. These techniques usually involve measurements of the irradiance and/or the polarization of the transmitted and reflected light from the sample under test. Common thin film measurement techniques include but are not limited to monochromatic reflectometry, spectral reflectometry, and ellipsometry.
In the method of monochromatic reflectometry, light is reflected off a sample, and the irradiances of both the incident and reflected beams are measured. The light spectrum is concentrated around a single wavelength, such as the light from a narrow bandwidth laser source. The incident light is of known polarization, which could be completely unpolarized or fully polarized, or some other known polarization state. With the known wavelength, known incident angle, and known refractive indices of the substrate and optionally the refractive index of the thin film coating, optical properties of the thin film coating, including the thickness and/or the refractive index of the thin film coating, can be determined.
In the method of spectral reflectometry, including spectrophotometry, various wavelengths are employed. One common setup of spectral reflectometry is to have a light source with variable wavelength, and change the light source wavelength in time series, and maintain the detector to be the same, as long as the detector is responsive to all the variable wavelengths. Another common setup of spectral reflectometry is to use a broadband light source, such as a white light source to directly illuminate the sample, and at the detector side, a spectrometer is used to direct light components of different wavelengths into different spectral channels. The irradiance reflectance of each wavelength is analyzed, and optical properties of the thin film coating could be calculated. Some spectral reflectometers are described in U.S. Pat. Nos. 7,126,131, 7,067,818 and 5,120,966, all incorporated by reference herein in their entirety.
Further, new methods in spectral reflectometry for application in ophthalmology are disclosed in the prior art, where the spectral reflection or reflected color as a result of the tear film interference is analyzed to retrieve optical thickness values. For example, U.S. Pat. No. 7,121,666, incorporated herein by reference in its entirety, describes a system that analyzes the lipid layer thickness, by comparing the dominant color of acquired image with a look up table, and the look up table is a simulation of color based on the setup in the patent. Also, U.S. Pat. Nos. 9,693,682 and 9,642,520, incorporated herein by reference in their entirety, describe a system that analyzes the tear film thickness. The system compares the RGB color of each pixel of acquired data with a tear film layer thickness (TFLT) palette, which is generated from theoretical optical wave interference model of the ocular surface, such as the air/SiO2/MgF2 or air/SiO2/MgF2/SiO2 model, or models with the refractive index and wavelength dispersion values of biological materials. The corresponding lipid layer thickness values are determined from the closest matching color in the TFLT palette. The thickness result accuracy in these patents is highly dependent on the accuracy of the theoretical model.
Furthermore, in U.S. Pat. No. 6,236,459, incorporated herein by reference in its entirety, a spectral reflectometric method with a plurality of wavelength filters for thin film analysis is described. A plurality of thin films with substantially predetermined intensity and spectral characteristics are used for calibration, and a look-up table comprising the data from a plurality of calibration thin films were used to enable accurate determination of the thin film thickness. Weight vectors based on orthogonal polynomials, such as normalized Legendre polynomials, are calculated in order to analyze the thin film thickness. This complicated calibration and analysis process requires precise fabrication and testing of a plurality of calibration thin films, and intensive mathematical calculation of weight vectors.
In the method of ellipsometry, the polarization states of the light before and after reflection or transmission of a sample are measured. The incident light can be monochromatic or broadband. Usually, polarizers, quarter-wave plates, or some other compensators are employed in the optical path. The incident angle and the wavelength spectrum of the light are properly chosen, and the orientation directions of the polarization elements are precisely controlled or rotated, so that the change in the polarization state of the beam could be measured. The change in polarization is characterized in both amplitude and phase changes, and they are very sensitive to the thickness and refractive index of the thin films. In the prior art of ellipsometers, two types are commonly seen, the rotating null ellipsometer and the rotating analyzer ellipsometer. Some types of ellipsometry methods or apparatuses are described in U.S. Pat. Nos. 5,166,752, 5,061,072, 4,957,368, 4,653,924, 4,647,207, and 3,985,447, all incorporated herein by reference in their entirety.
Conventionally, ellipsometry was limited to measure flat surfaces, and in recent years, several methods have been developed to apply ellipsometry on curved surfaces. Chao et al described a method of determining thickness of films on a curved substrate by a three-intensity measurement technique in “The ellipsometric measurements of a curved surface.” Japanese Journal of Applied Physics 44, no. 7L (2005), and “Determining thickness of films on a curved substrate by use of ellipsometric measurements.” Applied Optics 48, no. 17 (2009), incorporated herein by reference in their entirety. Furthermore, Li et al characterized a curved surface layer by Mueller matrix ellipsometry in “Characterization of curved surface layer by Mueller matrix ellipsometry.” Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena 34, no. 2 (2016), incorporated herein by reference in its entirety.
However, in the aforementioned prior art, these ellipsometric thin film measurement techniques are either limited to a flat sample, such as thin film coatings on a silicon wafer, or an approximately flat portion of a thin film coating on a curved substrate sample. For a curved surface, these techniques are either not applicable or only applicable to a very limited illuminated area of the measurement, where the region under investigation could be analyzed as a flat region. In some systems, an auxiliary focusing lens group is utilized to limit the illumination spot size to ensure the validity of the planar surface approximation of a small region on a curved surface.
Also, the prior art systems are limited to static coating, due to strict mounting constraints. Therefore, dynamic and fluidic coatings, such as the dynamically evolving lipid layer of a human tear film, anterior to a human eye, are generally not measurable with these techniques.
Further, to ensure a precise measurement with methods in the prior art, strict alignment requirements must be met. For example, the sample position has to be fixated and flat, the angles of incidence and reflection are precisely controlled, and the polarization elements precisely rotate at certain steps and to a certain position. Without a tight tolerance for alignment of these measurement systems in the prior art, the calculated thickness or refractive index values are not accurate, or even meaningless.
In some prior art systems, reference samples with known thin film coating thicknesses have been used for system calibration. For example, in U.S. Pat. No. 6,278,519, incorporated herein by reference in its entirety, a silicon substrate with an oxide layer of about 20 angstroms thickness is used for system calibration. However, these reference samples are also limited to planar or approximately planar samples.
Moreover, U.S. Pat. No. 9,615,735, incorporated herein by reference in its entirety, describes an optical coherence tomography system to measure the human tear film. However, that interferometer system requires a reference optical path to interfere the sample optical path with.